The invention relates to an electric incandescent lamp provided with a glass lamp vessel or envelope, which is sealed in a vacuum-tight manner, a lamp cap provided with contacts is secured to the lamp vessel in which a filament is arranged, which is electrically connected to contacts of the lamp cap.
The lamp vessel having opposite to the lamp cap a substantially hemispherical part, which has an axis of symmetry and has a reflective coating except in a window region near and around this axis.
The filament is arranged between a plurality of supporting points, around the axis of symmetry near the widest boundary of the reflective coating.
Such a lamp is known from GB 2,109,990 (Thorn EMI plc, 8 June 1983).
The known lamp has a filament, which is arranged in a flat plane which is at right angles to the axis of symmetry. The filament is situated within the hemispherical part of the lamp vessel.
The lamp of the kind mentioned in the opening paragraph is intended to be used in, for example, an external paraboloidal reflector. Light thrown by the filament onto the reflective coating of the lamp vessel is reflected to the external reflector and is concentrated by the latter to a light beam together with light thrown directly onto the external reflector by the filament.
In the hemispherical wall portion of the lamp vessel, the known lamp has a region not provided with a mirror-coating, which has for its object to keep the temperature of the lamp cap as low as possible. If this region should also be provided with a mirror coating, thermal and luminous radiation thrown onto this coating would in fact be reflected to the lamp cap.
The known lamp is suitable to be used at areas at which by a high luminous intensity the attention should be drawn to an object. However, the known lamp has proved to be not particularly suitable for applications in which stringent requirements are imposed on the shape of the light beam formed by the lamp together with an external reflector. Traffic signals form such an application. It has in fact been found that in the known lamp the position occupied by the filament with respect to the reflector is particularly critical. A forward or backward displacement of the filament of a few tenths of a millimeter with respect to the focus of the external reflector is already inadmissible in this lamp when used as a traffic signal lamp, unless the lamp consumes a higher power than is necessary in case of a correct positioning of the filament.
The particularly small tolerance in the position of the filament with respect to the external reflector requires, when used as a traffic signal lamp, an extremely accurate mounting of the filament in the lamp vessel. However, it is thus not guaranteed that the desired result is obtained. Fatal deviations from the correct position of the filament with respect to the external reflector may still be obtained due to the fact that the lamp is screwed more or less firmly into the lamp holder. However, also with the use of a Swan lamp cap and Swan lamp holder, such deviations may be obtained in case of a wrong positioning of the lamp holder with respect to the reflector. Moreover, it has been found that the known lamp has a short life.